By Vic Shelton
I don’t believe we think enough about soil erosion unless it’s keeping us from doing something. You can’t help but notice major rills or gullies in a field. You fix those so you don’t have to farm around them next time.
The more common “silent” erosion — that quietly removes a thin layer of soil from the surface slowly and steadily — is detrimental to long-term productivity and sustainability. We can build back soil organic matter slowly over time, but we can’t build soils back. Once they’re gone, they are gone.
Is there an amount of soil erosion that is “tolerable?” With today’s biology-building soil health systems, such as cover crops and never till, we should stop thinking about “tolerable soil loss” and start concentrating on “soil building.” We should be thinking about soil regeneration, not the amount we can lose and stay “sustainable.”
How erosion happens
When rain falls from the sky, it will do one of two things: land on soil or land on whatever is covering the soil. On bare soil, you can get splash erosion from the impact, which then joins other raindrops and starts to move the soil. There is quite often very little vegetation to slow it down.
However, if the raindrop hits vegetation, it breaks the rain’s impact, reducing runoff and improving infiltration. The first erosion is vertical erosion that fills the micropores and creates crusting.
Stabilizing and soil-improving fibrous roots from live plants such as cover crops stabilize the soil and can help build back lost organic matter. As soil organic matter and tilth improves, so does infiltration. Cover left on the soil surface certainly reduces the raindrop impact, but it also improves infiltration of that raindrop. The water infiltration rate of a continuous no-till field with 80% residue cover is about four times the infiltration rate of a tilled field.
Except for soils where drainage is an issue, if the soil is functioning properly, water should move downward through the profile except for the amount that is stored. A typical soil should contain about 50% pore space occupied by about half water and half air by volume at normal soil moisture. Soil particles (sand, silt and clay) make up the remaining 50%.
Soil compaction factor
Compaction is usually the primary reason that water doesn’t move downward through the soil profile correctly. Compaction changes structural characteristics and functionality of the soil. When you press together soil aggregates, the pore spaces between them are reduced. The ability of the soil to hold and store water for later is greatly reduced because of lost pore space.
Because the water can’t infiltrate downward efficiently, we see increased runoff even with good vegetative retardance. Maintaining cover with live growth year-round improves the soil over time, especially when you don’t break that trend with tillage. Those roots add organic matter to the soil over time, enhance the soil biology and break down compacted layers, improving aggregation and natural tilth of the soil. One tillage pass can erase years of progress.
Shelton is a grazing lands specialist with the Natural Resources Conservation Service in Indiana.